Best Skins Ever,
Invisible Shield, or
Body Guardz.....those will protect from scratches if you want something a little better i would recommend a silicone/rubber skin....check out these
Switch Easy,
iSkin Revo (Revo 2 for the iPhone 3G version) or.... go on the search for something under the
Accessory Review Topic and read some reviews on skins/hard cases/silicone/rubber cases and make a decision on which one sound best for your needs
P.S. if this post helped you, you can thank me

and it's always appreciated!